Ginny Weasley

The first time we meet Ginny is when Harry boards the Hogwarts Express in his first year. Ginny is overheard begging to go to Hogwarts, and then she is seen running alongside the train. We do not hear from her again until the second novel, in which she undertakes the key role of bringing Tom Riddle back to life via the enchanted diary. It is interesting that Rowling decides to use Ginny as the "victim" of the Chamber of Secrets, because the readers know next to nothing about Ron's baby sister. Of course, perhaps Rowling merely used Ginny as the connecting factor between the CoS and Ron. Had Ginny not been taken as a victim, I find it doubtful that Harry and Ron would've escorted Lockhart to the basilisk's lair.

Ginny appears throughout the third and fourth books sporadically  She is a lot like Neville Longbottom's character, in the sense that the readers come to know her personality and her character, but there is no real value placed upon her. It isn't until the fifth book when Ginny begins to date a few Gryffindor boys that the audience is given a glimpse into the potential of this pureblood witch. We learn that she is highly adept at hexes (the Bat Bogey hex in particular), and that she's is quite a bit like her twin brothers in the sense that she's a bit mischievous and very loyal.

However, the most intriguing aspect of Ginny is the relationship that develops between herself and Harry. Initially he sees her as nothing more than Ron's little sister, and as the years progress, he comes to view her as a friend. But in HBP, Harry starts to develop romantic feelings for Ginny, and the readers are taken along on their whirlwind relationship.

I firmly believe that had Ginny been kept as just Ron's little sister, the series would have ended very differently. Harry, who by all accounts is not a normal boy, would never be able to experience the very normal event of having a crush at school. Ginny plays an important role in the last few books, and had she not been such a brave and rebellious character in Hogwarts, Harry would never have been able to complete his task with the Horcruxes. We see this girl go from a virtual placeholder in the first novel to becoming an adept dueler standing next to Harry Potter.